Chapter 35 ELECTRIC CIRCUITS

What you will learn: You will distinguish between parallel and series circuits and series-parallel combinations and solve problems dealing with them. You will explain the function of fuses, circuit breakers, and ground-fault interrupters (GFI’s) and describe ammeters and voltmeters.  

Why it’s important: Electrical circuits are the basis of every electrical device, from electric lights to microwave ovens to computers. Understanding circuits helps you to use them, and to use them safely.     

Chapter 23: Series and Parallel Circuits I. A Battery and a Bulb (35.1) A. Circuit- A complete pathway for electrons to flow 1. Flow of electrons like flow of water in a closed system of pipes a. The battery would be analogous to the pump b. Wires analogous to the water pipes

B. The water flows through the pump and the electrons flow through the battery

II. Electric Circuits (35.2) A. Electric Circuit- any path along which electrons can flow 1. Must be a complete circuit with no gaps 2. Gap usually provided by electric switch

B. Most circuits have more than one device that receives electrical energy. Devices can be connected in a circuit in one of two ways, series or parallel.

III. Series Circuits (35.2) A. Electric current has but a single pathway through a series circuit. Current is the same through each electrical device in the circuit  

B. The total resistance to current in the circuit is the sum of the individual resistances along the circuit path R = RA + RB + …..  

C. The current in the circuit is equal to the voltage supplied by the source divided by the total resistance of the circuit (in accord with Ohm’s law)  

D. The total voltage impressed across a series circuit divides among the electrical devices in the circuit so that the sum of the “voltage drops” across each device is equal to the total voltage supplied by the source.  

E. The voltage drop across each device is proportional to its resistance. This follows from the fact that more energy is wasted as heat when a current passes through a high-resistance device than when the same current passes through a device offering little resistance.  

QUESTION: What happens to current in other lamps if one lamp in a series circuit burns out? ANSWER: The path to the current will break and current will cease. All lamps will go out

QUESTION: What happens to the light intensity of each lamp in a series circuit when more lamps are added to the circuit? ANSWER: This results in greater circuit resistance. This decreases the current in the circuit and therefore in each lamp, which causes dimming of the lamps. Energy is divided among more lamps……

QUESTION: What is the current through this series circuit? ANSWER: Use the equation for Ohm’s law and solve. First calculate the total resistance.

QUESTION: In this simple series circuit, a small electronic component is connected before the LED light bulb. What is this and why do you think they use it? ANSWER: The component is a resistor. It produces a voltage drop in accordance to Ohm’s law

IV. Parallel Circuit (35.4) A. Each device connects the same two points A and B of the circuit. The voltage is therefore the same across each device  

 B. The total current in the circuit divides among the parallel branches. Because the voltage across each branch is the same, the amount of current in each branch is inversely proportional to the resistance of the branch  Remember that the voltage is the same across each of the branches

C. The total current in the circuit equals the sum of the currents in its parallel branches   Where IA, IB, IC are currents through the branches and I is the total current

D. As the number of parallel branches is increased, the overall resistance of the circuit is decreased (just as more check-out cashiers at a supermarket decreases people-flow resistance). With each added parallel path,the overall circuit resistance is lowered. This means the overall resistance of the circuit is less than the resistance of any one of the branches. 

QUESTION: What happens to the current in other lamps if one of the lamps in a parallel circuit burns out? ANSWER: In one lamps burns out, the other lamps will be unaffected. The current in any other single branch is unchanged. The total current in the overall circuit is decreased by an amount equal to the current drawn by the lamp in question.

QUESTION: What happens to the light intensity of each lamp in a parallel circuit when more lamps are added in parallel to the circuit ANSWER: The light intensity for each lamp is unchanged. Only the total resistance and the total current in the total circuit changes. No changes in any individual branch in the circuit occur

V. Schematic Diagrams (35.5) A. Schematic diagram- simple diagrams to represent electrical circuits. 1. Symbols used to represent certain circuit elements 2. Circuit diagrams (schematics) show electrical connections, not the physical layout

3. Common Symbols:

B. Examples of Series and Parallel circuits

VI. Combining Resistors in a Compound Circuit (35.6) A. Sometimes it is useful to know the equivalent resistance of a circuit that has several resistors in its network 1. Equivalent resistance- value of the single resistor that would comprise the same load to the battery or power source 2. Calculate using the rules for adding resistors in series and parallel

B. Series circuits-

C. Parallel circuits-

VII. Parallel Circuits and Overloading (35.7) A. When add more devices (pathways) in house, the combined resistance is lowered in the circuit 1. Therefore, greater amount of current occurs 2. Can overload circuit and may result in heat (fire) B. Connect fuses in series along supply line to protect (fuse or circuit breaker prevents overloading)

Ammeters and Voltmeters An ammeter measures current in any branch or part of a circuit It does not change the current in the circuit

Voltmeter Used to measure voltage drop across some part of a circuit Has very high resistance so that is causes the smallest possible change in current or voltages in the circuit  

conventional current flow theory an older theory stating that electric current flows from the more positive source to the more negative source